It is highly desirable to produce copolymer of maleic anhydride and alkyl vinyl ethers which have a stable specific viscosity and which are not accompanied by the presence of potentially harmful solvents.
Copolymers of maleic anhydride and alkyl vinyl ether have a variety of important commercial uses, in the medical, cosmetic, pharmaceutical, food and other industries. One important property of the copolymer for these applications is its molecular weight, as commonly measured by its specific viscosity of a 1% (wt./vol.) solution in methyl ethyl ketone at 25.degree. C. For example, U.S. Pat. No. 3,003,988, teaches that a useful denture stabilizing composition is obtained from such copolymers having a specific viscosity exceeding 1.2, and observed good results with copolymers exhibiting a specific viscosity between 2.5 and 3.5. In many processing methods in converting the copolymer to useful consumer products, the copolymer can be exposed to temperatures of 25.degree. C. to 90.degree. C. for several hours during processing, for example, to manufacture the referenced denture stabilizing composition. The copolymer must maintain the desired specific viscosity during these operations at elevated temperatures. For many of the applications noted above, the benzene content of the copolymer is also an important property.
Production of interpolymers of methyl vinyl ether and maleic anhydride are generally conducted by dissolving the reactants, along with a free radical initiator, in a material which is a solvent for the reaction mix, which has no undesirable reactions with any component of the reaction mix, and which does not interfere with the free radical polymerization reaction which builds molecular weight. In addition, the material chosen as a solvent for the reaction mix is preferably a nonsolvent for the polymer product formed. The polymer thus becomes a slurry of finely divided particles as formed during the reaction, and can be separated easily from the solvent.
Maleic anhydride-alkyl vinyl ether copolymers have historically been produced in an aromatic solvent, such as benzene, as disclosed in U.S. Pat. Nos. 2,782,182 and 3,532,771. Product produced in a benzene solvent is inherently of stable shelf life in terms of viscosity, and exhibits no viscosity degradation when exposed to temperatures of 30.degree. to 90.degree. C. Unfortunately, benzene is a known carcinogen, and the residual benzene content of copolymer produced by this method limits the use of the copolymer in many products. However, it is very difficult to identify alternative solvents for the polymerization reaction which do not interfere with the chain growth mechanism of the polymerization reaction, and thus which are suitable for producing the desired high molecular weight copolymer product. Other solvents which are known to one skilled in the art include toluene, methylene chloride, and acetone.
U.S. Pat. No. 4,948,848 discloses a process for copolymerization of maleic anhydride and alkyl vinyl ether in acetone or ethyl acetate solvents. This procedure still produces a product containing extraneous solvents, and further requires a long time period for continuous feed of maleic anhydride dissolved in the solvent. Furthermore, to maintain the reaction product in a pumpable form, toluene was added. Toluene is commonly contaminated with small amounts of benzene. This patent also discloses the use of maleic anhydride dissolved only in methyl vinyl ether as a reactor feed, but observes that this produces undesirable results. Among other disadvantages, this patent observes that the viscosity of the copolymers cannot be controlled effectively.
U.S. Pat. No. 5,047,490 discloses a method for copolymerizing maleic anhydride and alkyl vinyl ether in an excess of the alkyl vinyl ether and without use of an extraneous solvent. However, although this reference can produce a copolymer of high molecular weight, the reference does not address the problem of long-term storage stability or degradation of product viscosity at slightly elevated temperatures of 30.degree. C. to 90.degree. C.
U.S. Pat. No. 4,370,454, discloses the use of a powder bed reactor to produce a copolymer of maleic anhydride and alkyl vinyl ether and/or an alkene. These copolymers are used for crust inhibition, dispersions, and the like, and generally are not of a controlled molecular weight, and many different low molecular weights are often suitable. When high molecular weights are desired, they teach the use of cross-linking agents. They do not recognize the importance of viscosity stabilization, and provide no teaching as to methods to accomplish this. It is desirable to provide a means of control of molecular weight to provide the capability to achieve high molecular weight copolymer without undesirable cross-linking agents and to provide a means of stabilizing the copolymer without the addition of foreign substances. The '454 reference does not recognize the need to accomplish these objectives, and do not teach one skilled in the art methods to accomplish these features.
A method has been disclosed in British Patent 906,230 for producing a stabilized vinyl alkyl ether-maleic anhydride interpolymer. This patent discloses that the interpolymer undergoes substantial and serious decomposition which is manifested by a decrease in viscosity over time, particularly after processing of the interpolymer at elevated temperatures. An object of that reference is to provide interpolymer compositions which are stable against degradation and/or decomposition, accomplished by the addition of free radical inhibiting compounds, such as phenols and aromatic nitro compounds. For many applications, this approach is unacceptable, due to the deleterious impact of adding these undesirable compounds to copolymers to be used in health and food applications, for example.